It is known that oxide semiconductors of specific compositions can provide resistive memory characteristics. More specifically, to explain the property of the oxide semiconductor, the oxide semiconductor exhibits, for example, a relatively high resistance in the initial condition between a pair of electrodes in contact with the oxide semiconductor. However, when a voltage of not less than a predetermined value is applied, the resistance is changed to a low resistance state, and this low resistance state is held (stored) even when the voltage is eliminated. On the other hand, in the case of the low resistance state, when a voltage of not less than a predetermined value is applied in the reverse direction between the pair of electrodes, the resistance is returned to the high resistance state, and the high resistance state is held (stored) even when the voltage is eliminated.
This type of oxide semiconductor can be switched between a low resistance state and a high resistance state by applying a voltage of not less than a threshold value in each of a forward direction and a reverse direction between a pair of electrodes, and this switching allows the resistance to be changed and stored. The use of the resistive switching characteristics allows the resistive switching memory devices to be used not only as resistive memory elements but also as switching elements.
The oxide semiconductors as described above are disclosed in, for example, Non-Patent Documents 1 and 2. Non-Patent Document 1 discloses a device including an n-type semiconductor and an electrode which forms a Schottky barrier, which develops a rectifying property and substantial hysteresis in current-voltage characteristics. Non-Patent Document 2 discloses a device which has a capacitor structure including p-type Pr0.7Ca0.3MnO3 and an Ag or Pt electrode, and provides resistive switching characteristics.
Besides Non-Patent Documents 1 and 2 described above, a large number of resistive switching memory devices each including an oxide semiconductor have been proposed, which are broadly divided into the resistive switching memory devices for developing the characteristics as described above, derived from the interface between the oxide semiconductor and an electrode (interface type or Schottky type), and derived from the bulk of the oxide semiconductor (bulk type or filament type).
In the case of constituting a resistive switching memory device with the use of the oxide semiconductor, a structure is typically adopted as shown in FIG. 14.
Referring to FIG. 14, a resistive switching memory device 1 includes an oxide semiconductor 2, and at least a pair of electrodes 3 and 4 opposed to each other with at least a portion of the oxide semiconductor 2 interposed therebetween. In this embodiment, the resistive switching memory device 1 has a capacitor structure with the lower electrode 4 formed on an insulating substrate 5, the thin-film oxide semiconductor 2 formed thereon, and further the thin-film upper electrode 3 formed thereon.
This resistive switching memory device 1 is of, for example, the interface type or Schottky type described above, in which at least one of the electrodes 3 and 4 is made of a material which can form a Schottky barrier which can develop a rectifying property and resistance change characteristics in an interface region between the electrode and the oxide semiconductor 2. The Schottky barrier has not only a rectifying property but also substantial hysteresis in the current-voltage characteristics, and the resistance state such as a high resistance state or a low resistance state can be changed by varying the polarity of the voltage applied between the electrodes 3 and 4.
To explain more specifically, for example, when the electrode 3 is assumed to form a Schottky barrier in an interface region between the electrode 3 and the oxide semiconductor 2, the interface region between the electrode 3 and the oxide semiconductor 2 is turned into a lower resistance in the case of applying a control voltage in a first direction between the pair of electrodes 3 and 4 through a pair of terminals 6 and 7, and then, this low resistance state is held even in the case of removing the control voltage in the first direction. On the other hand, in the case of applying a control voltage in a second direction opposite to the first direction between the pair of electrodes 3 and 4 through the pair of terminals 6 and 7, the interface region between the electrode 3 and the oxide semiconductor 2 is turned into a higher resistance, and then, this high resistance state is held even in the case of removing the control voltage in the second direction.
When not only a memory application but also an application as a resistive switch or impedance switch which has a memory function are considered as uses of the resistive switching memory device 1, the application of the control voltage described above is essential to change the resistance state. In particular, in the case of the resistive switching memory device 1 derived from a Schottky barrier, it is expected that new devices can be achieved by adding the resistance change and memory characteristics to p-n junction devices, diodes, and the like, currently in common use.
In the case of common diodes and p-n junction devices, the resistance value of the device can be changed substantially with a voltage applied, and the resistance value is reversed when the voltage is removed. Thus, the devices have no particular adverse effects as long as the devices each have two terminals in the same way as common resistors.
In contrast, in the case of the resistive switching memory device 1, it is possible to hold the resistance state thereof even when the voltage is removed. Thus, there is a need to change the resistance state by applying any voltage (control voltage), and then apply a voltage (driving voltage) for applying a current or a signal. In this case, there is a need to apply both the control voltage for switching the resistance state and the driving voltage for applying a current or a signal through the common terminals 6 and 7 in the resistive switching memory device 1 which has only the two terminals of the terminals 6 and 7 as shown in FIG. 14.
Therefore, in the case of applying the control voltage for changing the resistance state between the terminals 6 and 7, there is a need to temporarily disconnect the terminals 6 and 7 from the current line or the signal line with the use of a switch or the like, and the control of the resistive switching memory device 1 thus becomes complicated.
In addition, the resistive switching memory device 1 develops the resistive switching characteristics, and the rate of resistance change or the resistance value has a voltage dependence. Depending on the applied voltage, problems may be encountered, such as an excessively high or low resistance value, or an excessively low rate of resistance change.
In order to solve the problems as described above, it is desired to make it possible to control the resistance state independently.    Non-Patent Document 1: T. Fujii, and five others, “Hysteretic current-voltage characteristics and resistance switching at an epitaxial oxide Schottky Junction SrRuO3/SrTi0.99Nb0.0103”, APPLIED PHYSICS LETTERS 86, 012107 (2005)    Non-Patent Document 2: A. Odagawa, and five others, “Colossal electroresistance of a Pr0.7Ca0.3MnO3 thin film at room temperature”, PHYSICAL REVIEW B 70, 224403 (2004)